Impact printers of the dot matrix type are typically comprised of a print head supported upon a reciprocating carriage which moves across a stationary platen supporting a print receiving medium to effect printing. A print head is typically comprised of a housing for reciprocally mounting a plurality of print wires. The forward tips of the print wires extend through a jewel bearing at the nose of the print head while the rearward ends are driven by electromagnets to rapidly accelerate the print wires so as to achieve a velocity sufficient to impact an inked ribbon and transfer the ink therefrom to the print receiving medium. Although print heads of the type described are formed of highly reliable precision machined and/or precision molded parts, print heads nevertheless have a limited useful operating life, typically of the order of 15-20 million characters, and require periodic replacement. In addition, it is also desirable to be able to remove print heads for maintenance and inspection purposes and to gain access to surrounding components and structures, as well as the print heads themselves. In addition, it is highly desirable to increase the versatility of printers by the substitution of one type of print head for another. For example, print heads, although having the same basic overall design, may be provided with five, seven or nine print wires, for example, in order to print characters of lesser or greater resolution. The simplest way of obtaining these changes of character resolution is by direct substitution of a nine-wire print head for a seven-wire print head, for example. In the past, it has been conventional to mount print heads directly upon the carriage assembly by conventional fastening means such as threaded fasteners which pass through openings provided in the print head and threadedly engage tapped openings provided in the carriage. This arrangement is slow and tedious and further requires that the operator has sufficient skill to assemble and disassemble the aforesaid printer components.
In order to simplify the mounting and removal of print heads upon their carriage assemblies, a mounting bracket assembly has been developed and is described in detail in copending application Ser. No. 974067 filed Dec. 28, 1978, now U.S. Pat. No. 4,286,888 and assigned to the assignee of the present invention. The mounting bracket of the aforesaid pending application is secured to the print head by conventional fastening means and is provided with a pair of spring-loaded pins having enlarged heads which cooperate with a pair of slots provided in the carriage assembly for slidably mounting the print head upon the carriage assembly. A handle assembly which is capable of being swung between a locked and an unlocked position, and which is mounted upon the carriage, is initially maintained in the unlocked position to slide the print head and its cooperating mounted bracket on to the carriage assembly. The handle assembly is then swung to the locked position to secure the print head on the carriage. The handle assembly is formed of a resilient metallic member which exerts a biasing force upon the print head to normally urge the print head toward the platen. Rotatable cam means are provided along the forward end of the carriage to limit movement of the print head in the direction of the platen to provide a forms thickness adjustment.
The mounting assembly described in the aforementioned copending patent application is both bulky and heavy and requires that sufficient clearance be provided in the printer to slidably mount the print head upon the carriage assembly. In addition, it is not possible to provide precise forms thickness settings which may be repeatedly obtained without the need for additional adjustment each time a forms thickness setting is changed, thereby adding to the complexity of the forms thickness adjustment. Also, the mounting assembly requires a separate handle assembly mounted upon the carriage.